This invention relates to a tone-restoring apparatus by which when a recorded audio signal is reproduced at twice the recording speed, the tone at the reproducing time is restored to the same tone as at the recording time.
Recently, it has become important to reproduce an audio signal at a different speed from the recording speed. If the audio signal is reproduced at twice the recording speed, the reproducing time can be halved and therefore the contents of the recording on a tape recorded by a tape recorder or VTR can be grasped in half the normal time. However, if the audio signal is merely reproduced at twice normal speed, the tone becomes high and thus difficult to bear and to distinguish speaking persons. Thus, it is desired to produce apparatus by which the recorded speech can be caught in a short time with the tone not changed upon high-speed reproduction. Such an apparatus is disclosed, for example, in an article titled "Tape recorder capable to timebase compression and expansion of conversation" NIKEI Electronics 1976. 7. 26.
A conventional tone restoring apparatus will be described with reference to the drawings.
FIG. 9 is a block diagram of a conventional tone restoring apparatus. Referring to FIG. 9, reference numeral 1 represents an analog-to-digital converter circuit for converting an input signal to a digital signal, 2 a digital memory for storing the digital signal, 3 a write/read control circuit for controlling the digital memory 2 to write and read, 4 a hold circuit for holding a read signal from the digital memory 2, 16 a digital-to-analog converter circuit for converting the digital signal from the hold circuit 4 to an analog signal, 10 a modulation clock generator circuit for actuating the analog-to-digital converter circuit 1, 11 a write address generator circuit for supplying a write address to the digital memory 2 via the write/read control circuit 3, 12 a read address generator circuit for supplying a read address to the digital memory 2 via the write/read control circuit 3, 14 a demodulation clock generator circuit for actuating the digital-to-analog converter circuit 16, and 15 a low-pass filter.
The operation of this tone restoring apparatus will be described with reference to FIG. 10, which shows the principle thereof. It is assumed that when the reproduction is made at the same speed as at the recording, during the time t, 0.ltoreq.t&lt;2NT, N signals d0, d1, . . . , dN-1 are reproduced at a period of 2T. At this time, 2N signals d0, d1 . . . , d2N-1 are reproduced at twice normal speed during the time, t 0.ltoreq.t&lt;2NT. In order to restore this tone to the same pitch as at the recording time, the signals d0, d1, . . . , dN-1 are reproduced during the time t, 0.ltoreq.t&lt;2NT but the signals d.sub.N, d.sub.N+1, . . . , d.sub.2N-1 are not reproduced during that time, then signals d.sub.2N, d.sub.2N+1, . . . , d.sub.3N-1 are reproduced during the time t, 2NT.ltoreq.t&lt;4NT, and so on.
Thus, the reproduced signal is restored to the original pitch at the recording time.
In the above method, however, the signals d.sub.N-1 and d.sub.2N are discontinuous to each other and thus cause noise. In addition, the signals d.sub.N, d.sub.N+1, . . . , d.sub.2N-1 are not reproduced at all and therefore the information corresponding thereto is lost. If N is decreased to prevent continuous signals having long periods from being lost, the number of connection points per unit time increases resulting in an increase of noise.
Accordingly, it is an object of this invention to provide a tone restoring apparatus capable of reducing the loss of information and noise at connection points by using all the input signals and properly treating the discontinuous points.